The term "color display device" is used herein to designate a device that comprises three primary color light sources which form separation images in the three additive primary colors (red, green and blue) respectively. In the case of a shadow-mask color CRT, the three light sources comprise the respective electron guns and the associated phosphor deposits. A color display device receives a video signal having three primary color components (R, G, and B), and is adjusted such that a minimum valid value of any one of the three components (R, G and B) drives the corresponding light source to a minimum, or perceived off, condition and a maximum valid value drives the light source to maximum brightness. Typically, the minimum valid value is zero volts and the maximum valid value is 0.7 volts; and these values may be represented as 0 and 1 respectively in arbitrary units. The primary color components R, G and B are generally derived from encoded luminance and color difference components (e.g. R-Y and B-Y) using a resistive matrix. The Y, R-Y and B-Y components in turn are derived from a composite video signal, such as a signal in accordance with the NTSC or PAL format, using well-known filtering and demodulating techniques. For many years, the only significant source of a video signal was a video camera, which generates the video signal in primary color component form, encodes the primary color components into luminance and color difference components, and then combines the latter components to produce the composite video signal. Also, for many years most processing of the video signal took place in the primary color component domain or in the composite (NTSC or PAL) domain and video signals were not processed in luminance and color difference component form.
Since the values of the R, G and B components are independent variables, the range, or gamut, of colors that can be faithfully reproduced using a conventional color display device can be represented in a three-dimensional rectangular Cartesian coordinate system, having R, G and B axes, by a cube, as shown in FIG. 1. The eight corners of the cube represent the three additive primary colors, the three additive secondary colors (magenta, yellow and cyan), black and white. The solid and dot-dashed lines between the corners of the cube represent the transitions between colors of a standard color bar signal. In order for a color to be reproducible using a color display device, the point defined by the three color components of the target color must lie within the cube defined by the solid and dashed lines.
The conventional vectorscope provides a two-dimensional display of the color difference components R-Y and B-Y. The vectorscope display is luminance independent, and may be thought of as representing a projection of the FIG. 1 cube into a plane that is perpendicular to the (1,1,1) vector. Therefore, the primary and secondary colors are represented by the corners of a regular hexagon and the center of the hexagon represents both black and white. In FIG. 2, the solid lines and the dot-dashed lines between corners of the hexagon represent transitions between colors of a standard color bar signal. Any validly reproducible color, i.e. any color that can be represented by a point inside the color cube of FIG. 1, can also be represented by a point inside the hexagon defined by the solid and dashed lines of FIG. 2, but the converse is not true: not every point inside the hexagon of FIG. 2 corresponds to a point inside the cube of FIG. 1.
It has recently become common to generate composite video signals otherwise than from the primary color components. Such sources, e.g. television graphics systems, may generate signals directly in the luminance and color difference domain. Moreover, it has become common to process signals in the luminance and color difference domain. In Baker, "New and Unique Method for Measuring Video Analogue Component Signal Parameters" presented at the 19th Annual Winter SMPTE Conference held at San Francisco in February, 1985 and published in SMPTE Journal, October 1985, 1009, there is a discussion of a display format that is similar in some ways to a conventional vectorscope display but is particularly suited to a signal in luminance and color difference component format. The display described by Baker is a composite display of Y vs. R-Y and-Y vs. B-Y on alternate lines. The points representing the colors corresponding to the corners of the FIG. 1 cube are distributed in a zig-zag pattern (FIG. 3) down the display. A given color, defined by a set of values for R, G and B, is represented in this composite display by two points, one in the Y, R-Y space and the other in the -Y, B-Y space. As in FIGS. 1 and 2, the solid and dot-dashed lines in FIG. 3 between the primary and secondary color points represent transitions between the colors of a standard color bar signal.
The composite display described by Baker is particularly useful in observing timing and amplitude errors among the three components. It has been suggested that if a set of luminance and color difference values defines two points of which one lies inside the boundary defined by the solid and dashed lines in Y, R-Y space and the other of which lies inside the boundary defined by the solid and dashed lines in -Y, B-Y space, then that set of values defines a validly reproducible color.